1. Field of the Invention
The present invention relates generally to frequency generating devices for electrical circuits and more particularly to a piezoelectric micro-mechanical resonator that can be tuned to a specific frequency and switched between differing frequencies.
2. Description of the Related Art
Conventional oscillator design typically falls into one of two categories: quartz crystal controlled oscillators and non-crystal oscillators. The current standard for high precision frequency generation in electronic circuits is the quartz crystal oscillator. Quartz oscillators are desirable because they resist variations due to aging and temperature (among others), and because they typically possess extremely high quality factors. Quality factor (denoted as “Q-factor”, or simply “Q”) generally refers to a measure of an oscillator's resonance “sharpness.” Q is roughly defined as the ratio of height to width of the oscillator's resonant peak.
Typical quartz oscillators, however, cannot achieve the miniaturization levels desired by integrated circuit designers. Due to their size, quartz oscillators are constructed “off-chip” (i.e., separate from the integrated circuit that is being controlled). Additionally, quartz oscillators possess a limited frequency range and thus are not suitable for switching from one frequency to another (i.e., “frequency hopping”). Quartz oscillators also require large amounts of power to operate relative to other integrated circuit components.
An increased desire for integration, miniaturization, and power reduction has lead to a search for non-crystal oscillators that can be integrated “on-chip” and operated with the same power supplies as the circuitry that is being controlled. Non-crystal oscillators (which may be electrical circuits such as inductor-capacitor circuits and ring oscillators, among others) offer the ability to integrate the frequency source on-chip, with the accompanying advantages over multi-component quartz-based systems in size, power, frequency range and robustness to signal corruption through integration. Non-crystal oscillators, however, typically possess low Q-factors. For example, typical non-crystal oscillators have Q-factors on the order of tens, as compared to typical quartz oscillators whose Q factors are on the order of thousands. Thus, any circuit that is built with a non-crystal, on-chip frequency generator inherently possesses severely limited frequency precision and resolution.
One family of non-crystal oscillators utilize micro-mechanical resonators. Micro mechanical system technology and fabrication has given rise to many designs and applications of micro-mechanical resonators. In general, there are three types of basic micro-mechanical resonators: lumped-parameter (discrete spring-mass systems), flexural (beams, plates, and diaphragms) and acoustic wave (bulk and surface acoustic wave). Each of these resonators is used in a variety of applications such as sensors, oscillators, electromechanical filters, frequency references, high-speed signal processing, high-speed wireless communication systems, collision avoidance radar, intelligent transportation systems, scanned probe microscopy, touch sensitive probes, charge detection, gyroscopes, acoustic transducers and spectral analysis, among others.
Because resonant sensors themselves span such a variety of applications, further description is desirable. Typical micro-mechanical resonant sensors may be used in proximity sensors, strain gauges, pressure sensors, accelerometers, angular rate sensors, humidity sensors, fluid density sensors, and force sensors, among others. The resonant sensor's frequency output can be easily measured with digital electronics that result in high resolution results. Resonant sensors provide a compact, low cost, and more accurate substitution to their macro-sized counterparts.
In each of the above-mentioned applications, there is a desired frequency of operation, sometimes referred to as the “nominal frequency”, for the resonator. Thus in general, the above-mentioned resonators must have a “tuning” capability to achieve the desired resonance frequency. Tuning is used to compensate for effects caused by fabrication tolerances, residual manufacturing stresses, material defects, temperature effects, non-homogeneous material properties, material aging, thermal mismatch, contamination, and environmental factors, among others.
A need therefore exists for a resonator that has a higher Q factor than that offered by other electrical resonators, and which, unlike quartz resonators, can be fabricated on-chip to enable miniaturization, integration, and reduced power consumption. Additionally, a need exists for an improved resonator and a method of tuning or rapidly switching the frequency of the resonator which overcomes the limitations of quartz and non-crystal resonators and other limitations inherent in prior art resonators.